1. Field of the Invention
This invention relates generally to the measurement of the rate of fluid flow in a well and, more particularly, to determination, by ultrasonic means, of the rate of flow through various perforated intervals of a well casing.
2. Description of the Prior Art
In order to determine the rates of flow through various perforated intervals of a well, the rate of flow of material through the well casing is measured as a function of depth. Changes in the flow thus measured correspond to the amount of materials being added (or lost) within a given depth interval. Typically, such measurements are made with a "spinner" tool in which the RPM of a rotating impeller mounted on the tool indicates the rate of flow past the tool. Such systems are subject to several sources of error. First, the fluid flow through the impeller region of the tool may not be representative of the fluid flow through the casing as a whole. Secondly, mechanical fouling of the impeller and friction effects due to moving parts may cause erroneous readings. Finally, the impeller, at best, measures the rate of fluid flow past the tool. If the rate that the tool moves through the casing should vary, as if the tool sticks and is subsequently freed, errors will occur in interpreting the spinner data in terms of well flow.
The above problems may be solved in accordance with the present application by the use of a tool which incorporates an ultrasonic flow measuring device. Ultrasonic flow meters for determination of fluid flow velocity within a conduit are well known, but so far as Applicant is aware, have not been adapted to the measurement of variation in fluid flow rate with depth in a well. For example, as described in British Pat. No. 2,061,510, a determination is made of the difference in transit time between ultrasonic pulses transmitted upstream between a pair of transducers and transmitted downstream between them. The transducers are located on the wall of the casing in a fixed position. French Pat. No. 2,472,755 is another example of similar apparatus. British Pat. No. 2,066,466 describes the ultrasonic measurement of fluid velocity in a pipe by comparing the phase of transmitted and received signals on opposite sides of the pipe. U.S. Pat. No. 4,295,378 relates to the measurement of media flow in a pipe by the doppler frequency shift technique.
It is an object of this invention to provide an improved method and apparatus for measuring flow rate through various perforated intervals of a well which utilizes ultrasonic energy.
It is a further object of this invention to provide such a method and apparatus which further utilizes a doppler sonic flow measurement technique.
It is yet a further object of this invention to provide a method and apparatus of the character described which provides increased reliability and accuracy through elimination of moving parts.